The present invention relates to substrates coated with activated carbon useful for filtering impurities from fluid workstreams and, more specifically, to a method for uniformly activating carbon that resides on the walls of a honeycomb structure.
Activated carbon is a nongraphitic, microcrystalline form of carbon which has been processed to produce high porosity. Activated carbon is characterized by a high specific surface area (e.g., 300 to 2500 m2/g) and is known for its high adsorptive capability. The pores formed in the activated carbon may be macropores (i.e., pores having a diameter greater than about 500 angstroms), mesopores (i.e., pores having a diameter between about 20 and 500 angstroms), or micropores (i.e., pores having a diameter less than about 20 angstroms).
Activated carbon enjoys widespread use in the removal of impurities from fluid (i.e., liquid or gas) streams. For example, impurities in foods, fruit juices, and alcoholic beverages or medicinals (e.g., insulin, vitamins) can be successfully filtered using activated carbon. Likewise, activated carbon is useful in the removal of gaseous species present in low concentrations in air or gas streams such as gas separation processes, processes for removal of organic vapors, or in cigarette filters. Activated carbon has particular utility in adsorbing and purifying fluid emissions or workstreams from internal combustion engines.
Conventionally, activated carbon is used in a powdered or granular form. Powdered or granular activated carbon is inconvenient to use in processes where continuous workstream flows of fluids are filtered and/or treated. This is especially true for liquid fluids where tightly packed carbon beds cause significant pressure drop and pumps are required in order to maintain sufficient flow. To solve this problem, attempts have been made to use activated carbon in the form of, or in conjunction with, a solid substrate. For example, attempts have been made to manufacture monolithic substrates consisting essentially of activated carbon or to extrude carbonaceous material as a substrate and then convert the entire substrate to activated carbon. In such processes, a binder is typically added to the activated carbon powder and the mixture is extruded as a monolithic substrate. See, for example, U.S. Pat. Nos. 5,043,310 to Takeuchi, et al., 4,999,330 to Bose, et al., 4,399,052 to Sugino, and 4,386,947 to Mizuno, et al.
Substrates formed by these methods have limited utility. For example, the binder used to facilitate extrusion can block the pores of the activated carbon and, therefore, diminish the adsorption capability of the substrate. If the amount of binder is reduced to minimize blocking, the strength of the substrate is unacceptably reduced. Furthermore, most substances useful as extrusion binders begin to deteriorate at temperatures above 150xc2x0 C., further diminishing their applicability. Lastly, components of the process stream being filtered often react with commonly used extrusion binders, causing the binder to deteriorate during use. For example, water present in a fluid stream will dissolve methylcellulose, a very commonly used extrusion binder.
U.S. Pat. No. 4,518,704 to Okabayashi, et al. describes a method for making an activated carbon substrate using an inorganic binder (e.g., clay, talc, alumina, fusible glass powder). The high percentage of binder particles required to achieve acceptable strength in the honeycomb, however, results in low adsorptive capability. Furthermore, the strength of the formed substrate remains low due to the poor bonding of the carbon to the inorganic binders.
Other, equally unsatisfactory attempts to form carbon substrates feature coating a substrate with a slurry of carbon in a binder. See U.S. Pat. Nos. 4,992,319 to Kurosawa, et al. and 5,104,540 to Day, et al. The requisite binder in the carbon coating results in substrates with poor adsorptive capability due to the binder particles closing off some of the porosity in the activated carbon. Furthermore, the activated carbon is prone to flaking or chipping off of the substrate due to the weak bonds among the binder, the carbon, and the substrate.
Another problem that plagues the prior art designs for activated carbon honeycomb is the inability to uniformly activate the carbon channels. This is a significant problem when the honeycomb article has channels that are longer than the diameter of the individual channels. In these cases the gaseous oxidant is consumed prior to reaching the full length of the channels. Currently, the only way to address this problem is to increase reaction times and decrease the activation temperature This, however, is highly undesirable. Furthermore, even this approach will not work for honeycomb articles that have sealed or plugged ends.
Therefore it would be highly desirable to provide a honeycomb article for filtration or purification of workstreams that would have the following properties: a) excellent structural integrity, b) a low pressure drop through the filtration element, c) easy to manufacture at low cost, and d) uniformly high activity of the carbonaceous layer.
To achieve these desirable characteristics a process is required that allows for a rapid carbon-activating step that assures uniformity of activation.
In accordance with the desire to provide a structurally strong honeycomb article having a carbon filtration/purification element of uniform activity and a low pressure drop for process workstreams, the following process has been discovered. It is useful with substantially any honeycomb structure wherein the interior walls of the structure are composed of or incorporate a coating of carbon requiring uniform activation.
Briefly, the process utilized in accordance with the invention involves heating the honeycomb to release a carbon-activating material from a heat-activated source of carbon-activating material provided within the channels of the honeycomb. The heat-activated source material may be provided, for example, in intimate mixture with the thermosetting resin, and/or it may be introduced by filling the hollow regions of the honeycomb with carbon-activating material before or after converting the resin to carbon. The carbon-activating material is generally a material that, on heating, produces an oxidizing environment for the carbon layer, creating high-porosity activated carbon which uniformly resides on the walls of the honeycomb structure.